make external modules configurable

src/Statement.js

@@ -195,7 +195,7 @@ export default class Statement {
 
 					const declaration = statement.module.trace( subject.name );
 
-					if ( !declaration || declaration.statement.isIncluded ) {
+					if ( !declaration || declaration.isExternal || declaration.statement.isIncluded ) {
 						hasSideEffect = true;
 					}
 				}

test/function/configure-external-module-b/_config.js

@@ -0,0 +1,11 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'allows external module to be configured (b)',
+	options: {
+		external: [ 'path' ]
+	},
+	exports: function () {
+		assert.equal( require( 'path' ).resolve.configured, 'yes' );
+	}
+};

test/function/configure-external-module-b/main.js

@@ -0,0 +1,3 @@
+import { resolve } from 'path';
+
+resolve.configured = 'yes';

test/function/configure-external-module/_config.js

@@ -0,0 +1,11 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'allows external module to be configured',
+	options: {
+		external: [ 'path' ]
+	},
+	exports: function () {
+		assert.equal( require( 'path' ).resolve.configured, 'yes' );
+	}
+};

test/function/configure-external-module/main.js

@@ -0,0 +1,3 @@
+import * as path from 'path';
+
+path.resolve.configured = 'yes';